1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic disk for use in magnetic recording.
2. Background Art
Heretofore, a rigid magnetic disk has been used as one kind of magnetic disk. A rigid material having little flexibility has been used for the base of the magnetic disk. Generally, aluminum (for example, JIS A5086) has been used for the base of the rigid magnetic disk.
Such a rigid disk is generally produced by the following method. An aluminum disk is machined by a lathe and then its surface is ground so that it is possible to perform high-density recording with a reduced space between a head and the disk. Thereafter, a magnetic layer is formed on the disk by evaporating, spin coating, or the like. In this case, it is preferable that the surface of the disk is made as smooth as possible for the purpose of high-density recording/reproducing. In the conventional aluminum base disk, it is however difficult to obtain a surface having a center line average roughness Ra of not more than 0.1 micrometer. Further, when the magnetic layer is formed, there is such a limitation that webs cannot be applied continuously because of lack of flexibility of the base. Furthermore, since dust deposited on the surface has a great influence on high-density recording, the disk must be produced so as to be free from dust. Accordingly, the manufacturing process which requires much time is made more troublesome and more complex, resulting in the requirement of a large investment in equipment.
The conventional base is rigid and therefore inflexible as typified in an aluminum base. Accordingly, it is necessary to require that the head not come in contact with the magnetic layer of the disk when the head is tracing the magnetic layer. It is however difficult to maintain a constant narrow space between the head and the disk, so that error signals are often produced. Accordingly, it is very difficult to further reduce the space between the head and the surface of the disk in order to further increase the recording density. On the other hand, if the head accidentally touches the surface of the disk when the head is tracing the magnetic layer with such a narrow space between the head and the magnetic layer, a large impact force is likely to be concentrated at the contacting point of the magnetic layer because of the rigidity of the base so as to fracture the surface to thereby shorten the life of the disk.
There is such a further disadvantage that the aluminum base polished as described above is expensive.
To avoid the defects, recently, there has been proposed a magnetic disk arranged such that wide annular grooves are formed in opposite surfaces of a disk base. Two floppy disks or flexible disks (hereinafter generally referred to as "flexible disk sheets"), each having a magnetic layer on one of its surfaces are bonded on the disk base with the respective magnetic layers on the outside. As a result, a gap is maintained between the disk base and the rear surface of each of the flexible disk sheets.
The magnetic recording surfaces of the magnetic disk of this type is flexible, so that if a head accidentally touches the magnetic recording surface or even in the case where a head is intentionally caused to touch the magnetic recording surface to perform higher-density recording, the magnetic layer is hardly affected unlike the case of the rigid disk. Accordingly, the technique of flexible disks is applicable as it is. A durable magnetic layer having a smooth surface can be utilized as a magnetic layer for the magnetic disk of this type. Accordingly, the magnetic disk of this type has attracted special interest in that the defects of the conventional rigid magnetic disk can be avoided.